Wastewaters of many industrial plants (e.g. upgrading of uranium, production of fertilizers and explosives, nitro-organic compounds and pharmaceuticals) are nitrate bearing wastes. The use of nitrogen fertilizers and irrigation with domestic wastewaters are the main sources of nitrate pollution of groundwater in many developed and developing countries. Nitrate is one of the most problematic and widespread among the groundwater contaminants. The toxicity of nitrates to humans is due to the body's reduction of nitrate to nitrite that is related to clinical cyanosis (blue baby syndrome) and is a precursor of carcinogenic nitrosamines. Chronic consumption of high levels of nitrate may also cause other health problems, for example, some cancers and teratogenic effects. Due to the harmful effects of the nitrate ion, the European and US legislations have established the maximum admissible concentration for nitrates and nitrites in drinking water as 50 mg/l and 0.1 mg/l, respectively. The World Health Organization (WHO) recommends a maximum concentration for nitrate, nitrite and ammonium of 45 mg/l, 0.1 mg/l and 0.5 mg/l, respectively.
Most nitrate salts are soluble in aqueous medium so that nitrate ions are easily distributed to the groundwater sources. Standard water treatment practices, such as sedimentation, filtration, chlorination or pH adjustment with lime application, do not affect nitrate concentrations in water. Nitrates from contaminated groundwater can be removed by available physico-chemical methods for nitrate separation such as ion exchange, reverse osmosis and electrodialysis. In these processes the nitrates are concentrated in secondary waste streams which must be treated and thus result in high process costs.
A popular feasible method to solve the nitrate problem is biodenitrification. However, microbial denitrification processes are slow, sometimes incomplete and not easy to handle. Moreover, by direct biological denitrification the waters are intimately mixed with microbial cultures and organic compounds must be supplied as an energy source to drive the denitrification reaction. Residual organics may lead to other water quality problems.
Chemical reduction of nitrates can be conducted using various compounds, which are mainly hydrogen, iron, formic acid and aluminum. The main disadvantage of chemical reduction of nitrate is the production of additional wastes, which must be removed with a following treatment.
The catalytic hydrogenation of nitrates is viewed as a promising technology for removal of nitrates from polluted water. The hydrogenation process can be described by consecutive and parallel chemical equations 1a, 1b and 1c below:NO3−+H2→NO2−+H2O  (1a)2NO2−+3H2→(NO, N2O)→N2+2H2O+2OH−  (1 b)NO2−+3H2→NH4+−+2OH−  (1c)
These equations show that the nitrates undergo hydrogenation to nitrites and then to gaseous nitrogen (target product) and dissolved ammonia (undesired by-product).
A continuously performable process for the removal or reduction of the nitrite and/or nitrate content of nitrite-polluted and/or nitrate-polluted water with the selective formation of nitrogen by catalytic hydrogenation was first disclosed in U.S. Pat. No. 4,990,266. In U.S. Pat. No. 5,122,496, which is a divisional of U.S. Pat. No. 4,990,266, a catalyst is disclosed made from a porous inorganic carrier material, e.g. powdered alumina or silica impregnated with a metal component selected from palladium, rhodium, mixtures of palladium and rhodium, and mixtures of palladium and a metal of the copper group, preferably Cu. However, the use of powdered catalyst is restricted by a high-pressure drop in fixed beds and by the difficulty of separation of suspended powder catalysts. In addition, reduction of nitrate in water demands very active catalysts because the reaction has to be performed at the temperature of the groundwater (e.g. 25° C.). Moreover, a high selectivity is necessary to avoid the production of nitrite and ammonium ions by over-reduction of nitrite hydrogenation.
Several catalysts and supports have been investigated recently for catalytic denitrification of polluted water. Most of the hydrogenation catalysts (supported noble and transition metals) reduce nitrite mainly to ammonia. Both products (gaseous nitrogen and dissolved NH4+) were observed with Pd and Pt catalysts. Only supported Pd showed a high nitrite reduction activity and a low formation of ammonia. It was found to be a poor catalyst for the hydrogenation of nitrates, which was found to be accelerated by adding a metal promoter (Cu, Sn, In, or Zn) to Pd [Horold et al., 1993; Pintar et al., 1998; Vorlop and Prusse, 1999]. The efficiency of bimetallic catalyst depends on the ratio of the two metals [Kapoor and Viraraghavan, 1997] and on the catalyst preparation procedure [Vorlop and Prusse, 1999; Prusse et al., 1997; Batista et al., 1997]. The highest activity was observed for a Pd:Cu catalyst supported on powdered alumina (particles of 25 μm in size) with a ratio Pd:Cu of 4:1. The highest reaction selectivity was obtained for the bimetallic catalyst sample in which the copper was coated by a Pd layer [Batista et al., 1997]. Similar behavior was also reported for Pd—Cu sphere-shaped bimetallic catalysts with unimodal mesoporosity (pore radius of 3 nm) prepared by the sol-gel procedure [Strukul et al., 1996]. The obtained reaction selectivity was as low as 60-75%, probably due to inappropriate textural properties of these materials [Matatov-Meytal et al., 2000].
Significant effort has been devoted for preparation and optimization of catalysts for this process. It has been shown that while single noble metals (Pd, Pt) are active only with respect to nitrite hydrogenation, many supported noble metals with a metal promoter (Cu, Sn, In) show satisfactory performance for conversion of dissolved nitrate ions [Prusse and Vorlop, 2001]. The Pd—Cu catalyst suggested by Horold and Vorlop [Horold and Vorlop, 1993; U.S. Pat. No. 5,122,496] is still considered to be the best catalyst for this process.
It is known that the activity and selectivity of the bimetallic catalyst in nitrate hydrogenation is highly dependent on the preparation method, mode of noble metal promoting, metal-promoter ratio as well as operation conditions [Matatov-Meytal and Sheintuch, 2005]. The selection of the support material is also important and several materials have been proposed as support for Pd—Cu catalysts: silica and alumina [Vorlop and Prusse, 1999; Pintar et al., 1998], zirconia [Gavagnin et al., 2002], titania [Gao et al., 2003], polymers [Kralic and Biffis, 2001], granular active carbon [Yoshinaga et al.,2002; Lemaignen et al., 2002] and other materials [Daganello et al., 2000], and found to greatly affect the catalyst's activity and selectivity towards the reaction products. There is currently a growing interest in using novel structured supports like monoliths, foams, membranes as well as fibrous cloths [Matatov-Meytal and Sheintuch, 2005] as support for bimetallic catalysts. Cloths woven from thin μm-sized fibers reduce the diffusion distance and produce a low-pressure drop in fixed beds and in multi-phase reactors in which one or more dissolved species have to react with gaseous compounds of limited solubility. Moreover, cloth-type catalysts are preferable to monoliths when rapid fluctuations in the flow regime occur, as often-encountered in applications for environmental protection.
Recently, the inventors of the present invention have studied some novel Pd-based catalytic cloths for hydrodechlorination [Shindler et al., 2001] and for nitrite and nitrate hydrogenation [Matatov-Meytal et al., 2003; Matatov-Meytal and Sheintuch, 2005, 2009; Matatov-Meytal, 2005]. In this aspect, particular attention was given to activated carbon cloths, that have been proven to have a great potential as a catalytic support, especially for the expensive noble metals, since a high metal loading and dispersion can be achieved.
In spite of the many efforts that have been devoted to developing a catalyst for direct removal of nitrate and nitrites from nitrate-polluted water streams into nitrogen, stable catalysts with adequate both nitrate removal efficiency and selectivity towards nitrogen production have not yet been produced. The main reasons which inhibit the use of the catalytic hydrogenation as an effective water denitrification technology are the difficulty to modulate the activity of the existing catalysts and their selectivity to avoid nitrite and ammonium formation. Therefore, there still exists a need for a catalyst to convert nitrate and simultaneously nitrite into nitrogen gas